1. Field of the Invention
The present invention relates to an analog filter that is suitably used as a filter for smoothing a ΔΣ-modulated signal, for example.
2. Description of the Related Art
A ΔΣ-modulation is a method for encoding an analog signal with a variation in the data for the immediately preceding data at each sampling point sampled at each timing of a predetermined sampling frequency in converting an analog signal into a digital signal. That is, the ΔΣ-modulation represents an amplitude component of the analog signal in binary value (one bit) alone.
This ΔΣ-modulation is employed for encoding an audio signal, for example. The ΔΣ-modulation method has the merits that the overall constitution can be simplified as compared with the PCM method for the CD (Compact Disk) widely used at present, and the restorability from the digital signal to the original analog signal can be enhanced by controlling the distribution of quantization noise.
That is, in the PCM method, the analog signal is replaced with the digital signal by making an arithmetic operation based on the quantization characteristic at each timing of the sampling frequency, and the absolute amount of data are recorded at all the sampling points. On the contrary, in the ΔΣ-modulation method, a variation in the data for the immediately preceding data is only recorded, and no thinning or interpolation of information amount is made, unlike the PCM method, whereby the binary signal produced by quantization presents a characteristic quite close to analog characteristic.
Accordingly, in the case of reproducing the digital signal encoded on the basis of the ΔΣ-modulation method, the original analog signal can be reproduced by making a simple process of removing high frequency components of the digital signal through a low pass filter provided at the last stage, without need for a D/A converter, unlike the PCM method. In practice, in the conventional audio reproducing apparatus, the original analog signal was reproduced by passing the ΔΣ-modulated signal through the low pass filter.
However, when the low pass filter was used, there was a problem that the output waveform was distorted due to a phase distortion of the low pass filter. Another method is conceived in which an interpolation process with the sinc function is made by applying an over-sampling technique currently employed for the CD and so on. However, since this sinc function converges to zero toward ±∞, a discretization error occurs in the arithmetic operation and a distorted output waveform is produced. Also, there was another problem that the constitution was quite complex.
The present invention has been achieved to solve the above-mentioned problems, and it is an object of the invention to provide an analog filter optimal for the ΔΣ-modulated output. Specifically, the invention is aimed at providing an analog filter with less distortion in the output waveform and of a simple constitution.